To investigate the potential relationship between month of birth and breast cancer risk, the authors examined the month of birth of 14,289 women with breast cancer and 38,151 women with cancer of sites other than the breast. All cases were newly diagnosed between 1974 and 1984, and were identified through the Cancer Surveillance System, one of 10 population-based cancer registries comprising the Surveillance, Epidemiology and End Results Program of the National Cancer Institute in the United States. Although breast cancer cases exhibited spring and fall peaks in their month of birth (confirming a previous report), the month of birth of non-breast cancer cases followed a similar distribution. Such patterns have also been reported from populations in other parts of the United States, western Europe, Japan and Australia. These findings suggest that the seasonal variation observed in month of birth of women who develop breast cancer is a reflection of the pattern of births in the underlying population, rather than a new variant in breast cancer epidemiology.